Die casting with magnesium has increased substantially due mainly to the extreme light weight of the finished parts. On the basis of equal volume, magnesium is approximately less than 1/4 of the weight of steel and 2/3 the weight of aluminum. It will be appreciated that light weight in castings is important for numerous reasons some of which are (a) the saving of power and increasing the efficiency of machines that have moving parts; (b) increased payload in the field of transportation. Products made from magnesium tend to dissipate heat much faster than other alloys and accordingly the use of magnesium in the castings of computer cases, RF shielding and the like is most desirable.
Magnesium ages well; it has the approximate weight of plastic with the strength of zinc and it is less expensive than zinc. Casting made from magnesium have good energy absorption qualities and can be machined at higher speeds and with greater economy than any of the other die casting alloys.
There are however inherent problems in the use of magnesium for die casting and these problems usually are centred around the fact that magnesium in a molten state is highly volatile and there is usually a substantial risk of burning and explosion if great care is not taken in handling the molten metal. Desirably, the bath or crucible with molten metal therein should be well sealed from the ambience but this often present problems in locating injection assemblies in the machine. A blanket of inert gas covers the top of the molten metal in the crucible where the metal is maintained in temperatures ranging from 1175.degree. to 1250.degree. F. for the purposes of injection.
Due to some of these inherent problems of die casting with alloys, such as magnesium, production rates have tended to be much lower than those used in the casting of other alloys such as zinc. It will be appreciated that in order to produce an economical die casting, substantial production rates must be maintained. The result of these inherent difficulties has been that, to a great extent, magnesium alloys have not been cast in high speed machines incorporating injection assemblies submerged in the crucible of molten metals.
Canadian Patent 564,340 of Oct. 7, 1958 discloses an apparatus for producing magnesium die castings and it utilizes a "straight" goose neck in which the upper limit of the molten metal in the upper end of the nozzle and the upper surface of the molten metal in the pot is substantially coplanar. An offset arm is pivoted for swinging movement of the complete pot and nozzle toward and away from the surface of the die. This patent utilizes a plunger valve control operated in a vertical sleeve.
The prior art examples mentioned above do not disclose the advanced aspects of the present invention which utilizes an injection pump mounted in a manner so as to clear the upper access of the crucible and facilitate cover sealing and removing for cleaning or purging operations. The prior art does not show an injection pump assembly which comprises two plunger systems, one acting as a spool valve to open and close the injection barrel passage and another one to inject the molten metal into a mold through a barrel passage.